Add basically API documentation

docs/concept/mail-blackwhitelist.md

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+# Mail (server) black- and whitelist
+
+## mail server blacklist
+
+### Problem
+
+- some mail servers don't accept mails due to their spam filter
+- users blame timelimit for it although their bad mail service is the problem
+
+### Solution
+
+- (external) monitoring
+- the blacklist
+
+### Description
+
+The mail server blacklist is a list of servers to which no sign up mails are sent.
+But instead of doing nothing, the client is informed about that so that it can tell
+the user that this mail service is not supported and that he should use a mail address
+at an other mail service.
+
+## mail (server) whitelist
+
+### Problem
+
+- some users do run private server instances
+- they do not want that someone else can use their instance
+
+### Solution
+
+- the whitelist
+
+### Description
+
+The whitelist contains mail servers and/ or mail addresses which should be allowed.
+Users of mail addresses which are not in the whitelist are informed about that
+so that they know that it is a private instance and that they can not use it.

docs/concept/premium.md

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+## Premium concept
+
+- some features are locked after some days
+- unlocking these features again requires a purchase
+- purchases can be added manually using the [admin API](../api/admin.md)

docs/concept/primary-device.md

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+# Primary device concept
+
+One (child) user can be assigned to multiple devices. Without synchronization,
+this can result in using the time limits once per device and not only once.
+Due to that, there is (by default) the limitation that a child must select
+one device for using limited Apps.
+
+There can be one or zero primary devices. This is required for the primary device
+switching process and for the initial state.
+
+## assign process
+
+- sync
+- try to assign the user using the [child API](../api/child.md)
+- if the user is assigned to a different device, request a logout using the [child API](../api/child.md) and retry it
+  - this should not be done in a loop without delays
+  - using the [websocket](./websocket.md) it is possible to get changes without polling
+  - requesting a sign out should be repeated all few seconds
+
+## unassign process
+
+- sync
+- unassign the user using the [child API](../api/child.md)

docs/concept/status-message.md

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+# Status message
+
+- clients should show the status message if there is one which is not an empty string
+- can be used to announce server maintenances
+- can set using the [admin API](../api/admin.md)